In the past, blade producers often had problems with their mould shells suffering small deformations that increased with the number of blades produced, gradually becoming major errors in the blade shape. These deformations are caused by cycles of heating and cooling while the mould is under some pressure or load. Essentially the mould shell slowly becomes stretched out of shape as many blades are produced. This deformation overwhelmingly occurs in the cross section shape of the blade mould, as the two spanwise edges are normally supported by the sturdy steel backing frame.
Some early attempts to correct the mould cross chordwise shape failed because the importance of adding spanwise oriented elements such as the laminated stiffening tubes was not recognized. Accordingly, the zone of induced deformation around each adjustment point was small in size and roughly circular in shape. It was found that an excessively small spacing of adjustment points in the spanwise direction was required to obtain a useful shape correction, and the whole effort at adjustment became expensive and complicated, finally unworkable.
Other early attempts to maintain the mould shape failed because the stiffening elements were aligned in the cross chordwise plane, rather than spanwise, e.g. as bulkheads made from wood, steel, etc. In some cases such bulkheads were also retained with threaded supports, giving some measure of adjustability. However such bulkheads only served to maintain the mould shape for a short distance in the spanwise direction, and the sections between each bulkhead were substantially unsupported. After long use, mould with such stiffening systems take on an unsatisfactory ‘corrugated’ appearance, owing to the sagging and unrecoverable deformation which inevitably occurs between each bulkhead.